Datasheet

Alarm Signaling
After the device performs a temperature conversion, the
temperature value is compared to the user-defined two’s
complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see T
H
and T
L
Register Format). The
signbit(S) indicatesifthevalueispositiveornegative;
forpositivenumbersS=0andfornegativenumbersS
= 1. The T
H
and T
L
registersarenonvolatile(EEPROM)
so they retain data when the device is powered down.
T
H
and T
L
can be accessed through bytes 2 and 3 of the
scratchpad, as explained in the Memory section.
Onlybits11:4ofthetemperatureregisterareusedinthe
T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers.
If the measured temperature is lower than or equal to T
L
or higher than or equal to T
H
, an alarm condition exists
and an alarm flag is set inside the device. This flag is
updatedaftereverytemperaturemeasurement;therefore,
if the alarm condition goes away, the flag is turned off after
the next temperature conversion.
The master device can check the alarm flag status of
all MAX31820 devices on the bus by issuing an Alarm
Search[ECh]command.Anydeviceswithasetalarmflag
respond to the command, so the master can determine
exactly which devices have experienced an alarm condi-
tion. If an alarm condition exists and the T
H
or T
L
settings
have changed, another temperature conversion should be
done to validate the alarm condition.
T
H
and T
L
Register Format
Powering the MAX31820
The device can be powered by an external supply on
theV
DD
pin, or it can operate in “parasite power” mode,
which allows the device to function without a local exter-
nalsupply.Parasitepowerisveryusefulforapplications
that require remote temperature sensing, or those that
are very space constrained.
Figure1 shows the device’s
parasite-power control circuitry, which “steals” power
fromthe1-WirebusthroughtheDQpinwhenthebusis
high. The stolen charge powers the device while the bus
is high, and some of the charge is stored on the parasite-
power capacitor (C
PP
) to provide power when the bus is
low. When the device is used in parasite-power mode,
theV
DD
pin must be connected to ground.
In parasite-power mode, the 1-Wire bus and C
PP
can
provide sufficient current to the device for most opera-
tions as long as the specified timing and voltage require-
ments are met (see the DC Electrical Characteristics and
AC Electrical Characteristics
tables).However,whenthe
device is performing temperature conversions or copy-
ingdatafromthescratchpadmemorytoEEPROM,the
operating current can be as high as 1.5mA. This current
can cause an unacceptable voltage drop across the weak
1-Wire pullup resistor and is more current than can be
supplied by C
PP
. To ensure that the device has sufficient
supply current, it is necessary to provide a strong pullup
on the 1-Wire bus whenever temperature conversions
Figure 1. Supplying the Parasite-Powered MAX31820 During Temperature Conversions
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
µP
V
PU
4.7kΩ
V
PU
1-Wire BUS
TO OTHER 1-Wire DEVICES
MAX31820
GND DQ V
DD
MAX31820 1-Wire Ambient Temperature Sensor
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